In an electric power network, so-called subsynchronous resonance (SSR) oscillations may arise by the cooperation between mechanical oscillations in, for example, turbogenerators connected to the network and the electrical system. The oscillations may, for example, have a frequency of 20 Hz. They may have positive feedback (negative damping) and reach such amplitudes as to damage the shafts of the generator units.
The SSR oscillation has proved to be particularly serious in such networks which include power lines with series capacitors for compensation of the line inductance and thereby increasing the transmission capacity of the lines.
In has been proposed to connect, in series with each phase in the block transformer of a turbo-group and/or the star point of the generator, an ohmic damping resistor in parallel with an inductor and with a series resonance circuit tuned to the power system frequency. This solution is known from the Swedish published patent application with publication number 446 289. These damping circuits may be permanently connected or alternatively be put into operation under the control of a disturbance detector. This device comprises a large number of components, which must be dimensioned for the maximum current of the line. Therefore, the device will be expensive and space-demanding. The desired damping of SSR is further obtained by the consumption of power in the resistors of the device. These must therefore be so amply dimensioned or so efficiently cooled that they are able to take care of the large energies which are consumed during a damping process without being overstressed.
From, for example, U.S. Pat. Nos. 4,292,545 and 4,434,376, damping circuits for SSR are known, which are used in series capacitors in power lines. With the aid of thyristor valves, damping resistors are connected into the network circuit, for example in parallel with the capacitors, when an SSR occurs. The connection is initiated by a detector which connects a damping resistor each time that the length of a half-cycle of the line voltage exceeds a predetermined value, preferably approximately equal to the length of a half-cycle at a power system frequency equal to the system frequency of the network. Also in this device a heavily dimensioned resistor is required, which is a clear disadvantage from economical and practical points of view.